1. Field of the Invention
This invention relates generally to a valve system for the hydraulic work system of a work vehicle, in particular an industrial truck, with an elevating drive system to raise and lower a load and a tilting drive system, each of which can be actuated by respective control valves, in particular by electrically actuated control valves, the opening width of which determines the speed of movement of the respective users, wherein the elevating drive system is realized in the form of a single-action hydraulic cylinder and a check valve that is realized in the form of a seat valve and opens toward the hydraulic cylinder and is located in a hydraulic line that leads from the control valve to the hydraulic cylinder, and wherein the tilting drive system is realized in the form of a double-action hydraulic cylinder.
2. Description of the Currently Available Technology
Valve systems of the type described above are used in industrial trucks, for example, and in fork lift trucks in particular. The elevating drive system, in the form of one or more lifting cylinders connected in series, is used to raise and lower loads. To raise the load, the lifting cylinder, which is realized in the form of a single-action hydraulic cylinder, can be connected by means of a control valve to a pump line, and for lowering the load it can be connected to a return line that is connected to a reservoir. In such a case, the control valve is realized in the form of a proportional valve, the opening width of which determines the quantity of hydraulic fluid that is admitted into the lifting cylinder or is discharged from the lifting cylinder, and thus determines its speed of movement. A pilot-controlled check valve in the hydraulic line that leads from the control valve to the lifting cylinder blocks off the lifting cylinder with no leakage of hydraulic fluid. In systems of the prior art, during the lifting operation, the check valve is switched into the open position by the pressure of the hydraulic fluid flowing into the lifting cylinder. For lowering, the check valve is unblocked, whereby the check valve is opened by the pressure in the pump line. In that case, however, to open the check valve to lower the load, a pressure must first be built up in the pump line that at least equals the load pressure of the elevating drive system to switch the check valve into the open position. That results in energy losses which, on battery-operated fork lift trucks in particular, lead to a reduced operating time of a charged battery.
When electrically actuated control valves are used, such as control valves actuated by electromagnets, for example, it must also be taken into consideration that the control valve for the elevating drive system cannot be opened too wide when a load is being lowered to prevent an excessively rapid descending movement and to guarantee that there is a sufficient braking distance available for the deceleration of the load. For this purpose, on electrically actuated control valves of the prior art, the opening width of the control valve is limited in the xe2x80x9cdescentxe2x80x9d position, which means that small loads are lowered at a slow speed of descent since, as the result of the presence of only a small pressure differential at the throttle point of the control valve, the speed of descent that can be used to lower a large load cannot be achieved.
The hydraulic cylinders of the tilting drive system are generally realized in the form of double-action hydraulic cylinders. To achieve a firm restraint of the hydraulic cylinders in tilting drive systems of the prior art, there is a pilot-controlled check valve in each of the hydraulic lines that leads to the hydraulic cylinders. The check valve located in the discharge line is thereby switched into the open position by the pressure in the respective admission line. However, such a system is susceptible to oscillations. To prevent oscillations, high restraining pressures of the check valves are set that must accordingly be overcome by the admission pressure to achieve a high stability. However, especially when the loads to be moved by the tilting drive system are small, there are large losses that lead to a shorter battery life in industrial trucks that are powered by rechargeable batteries.
Therefore, it is an object of the invention to make available a valve system of the type described above that has an improved function with low energy losses.
The invention provides a valve system in which the check valve has a control surface that acts toward the opening position, which control surface can be pressurized by the load pressure of the elevating drive system, and a control pressure chamber of the check valve that acts toward the closed position, which control pressure chamber can be pressurized with the load pressure of the elevating drive system and, when the control valve is actuated into the xe2x80x9cdescentxe2x80x9d position, can be placed in communication with a reservoir, and/or that there are flow regulators located in both the admission line and in the discharge line respectively of the control valve of the tilting drive system, which flow regulators have an open and a closed position, whereby the flow regulators are respectively pressurized toward the closed position by the pressure upstream of the throttle point of the control valve, and toward the open position by the pressure downstream of the throttle point of the control valve and by a spring.
The control pressure chamber of the check valve that is pressurized by the load pressure of the elevating drive system and is active in the closed position can thus be relieved when a load is lowered. The check valve is thereby opened by the load pressure of the elevating drive system that is applied to the control surface that acts in the opening direction. When the user is idle, the hydraulic line that leads to the lifting cylinder is blocked off by the check valve without any leakage of hydraulic fluid. When the load is lowered, the control pressure chamber of the check valve pressurized by the load pressure is thereby relieved and the check valve is actuated by the load pressure applied to the control surface that acts in the opening direction. No pressure needs to be built up in the pump line to open the check valve, and consequently there are no energy losses.
The hydraulic cylinders of the tilting drive system are restrained by means of respective flow controllers in the admission and discharge lines of the tilting drive system, whereby the admission-side flow controller is controlled by the pressure decrease at the admission-side throttle point, and the discharge-side flow controller is controlled by the pressure decrease at the discharge-side throttle point of the control valve. The flow controllers thereby maintain the incoming and outgoing flow of hydraulic fluid, regardless of the load, at the value specified by the opening width of the control valve. The tilting drive system can thereby be operated at the speed of movement specified at the control valve, independent of the height of the load. The tilting drive system is thereby restrained in both directions of movement between the flow regulators, whereby the only loss is the pressure drop that is necessary for the control of the flow regulators at the throttle points of the control valve.
The switching systems of the invention for the elevating drive system and the tilting drive system can be used individually or in combination with one another. Their combined use increases the efficiency of the overall system.
It is particularly advantageous if the control pressure chamber of the check valve can be placed in communication with a relief valve that is realized in the form of a seat valve and can be connected to a reservoir, whereby the relief valve can be moved into a closed position by the load pressure of the elevating drive system, and into an open position when the control valve of the elevating drive system is actuated into the xe2x80x9cdescentxe2x80x9d position, in which position the control pressure chamber of the check valve can be placed in communication with a reservoir. When the user is at idle, the hydraulic line leading to the lifting cylinder is therefore blocked off without any leakage of hydraulic fluid by the check valve and the relief valve which is realized in the form of a seat valve. During the descent, the control pressure chamber of the check valve, which is pressurized by the load pressure, can easily be placed in communication with the reservoir by means of the relief valve acting as a pilot valve, and the check valve can be actuated.
In one embodiment of the invention, the control pressure chamber of the check valve can be pressurized by means of a fixed diaphragm with the load pressure of the elevating drive system. When the elevating drive system is not actuated, therefore, the load pressure of the elevating drive system is applied to the control surface and in the control pressure chamber, as a result of which the check valve is held in the closed position. When the control pressure chamber of the check valve is depressurized by opening the relief valve, a pressure drop occurs at the diaphragm, as a result of which the pressure in the control pressure chamber of the check valve is lower than the load pressure of the elevating drive system being applied to the control surface. It thereby becomes possible to ensure in a simple manner that the check valve is deflected into the open position at the beginning of the descent movement.
The invention teaches that it is particularly advantageous if a stepper motor is provided as the actuator device of the control valve of the elevating drive system, whereby the stepper motor is effectively connected to the relief valve, and when the control valve is actuated into the xe2x80x9cdescentxe2x80x9d position, the stepper motor moves the relief valve into the open position. The stepper motor is actuated by means of a digital actuation signal, for example a number of control pulses, and converts the actuator signals into a position of the output shaft and thus a corresponding position of the control valve. As a result of the digital actuation, a precise gradation of the position of the output shaft of the stepper motor and a high accuracy of repetition can be achieved, as a result of which the control valve for the elevating drive can also be actuated with a high degree of precision and repeatability. As a result of the actuation of the relief valve into the open position by the stepper motor, when the control valve is actuated into a position to lower a load, the relief valve can be easily opened and thus the check valve can be actuated.
It is thereby particularly advantageous if the control valve of the elevating drive system is realized in the form of a longitudinal slide valve with a slide piston, and the stepper motor is connected to the valve slide of the control valve by means of a transmission, in particular a spindle-nut transmission, whereby the valve slide is secured to prevent rotation and is mounted so that it can move longitudinally in a housing boring, and whereby a spring device is provided that holds the valve slide in the middle position when the slide is not actuated. As a result of the presence of a spindle-nut transmission, it is easy to convert a rotational movement of the output shaft of the stepper motor into a translation motion for the deflection of the valve slide of the control valve. The spring device ensures that when the stepper motor is not actuated, the control valve is maintained in the middle position, and thus the elevating drive system is blocked off, with no leakage of hydraulic fluid, by the check valve and the relief valve.
The invention teaches that it is particularly advantageous if the relief valve has a valve body that is effectively connected by means of an actuator element to the valve slide of the control valve of the elevating drive system. The actuator element can be, for example, a pin located on the valve body of the relief valve, which pin is effectively connected with the valve slide. When the stepper motor is actuated and thus there is a deflection of the valve slide into the descent position, it thereby becomes possible to easily move the relief valve into the open position.
In one embodiment of the invention, in the return line that runs from the control valve to the tank line, there is a descent braking valve that can be pressurized toward an open position by a spring and by the pressure downstream of the control valve, and toward a closed position by the pressure upstream of the control valve. During the descent, a defined speed of descent is specified by the opening width of the control valve. The descent braking valve thereby controls the speed of descent regardless of the load being exerted on the elevating drive system. For this purpose, the descent braking valve is controlled by the pressure decrease that occurs at the discharge-side throttle point of the control valve. For a small load, there is a small decrease in pressure at the control valve, as a result of which the descent braking valve is held in the open position. For a large load, and thus a high load pressure upstream of the control valve, there is a large decrease in pressure at the control valve, as a result of which the descent braking valve is pressurized toward the closed position, thereby preventing an increase in the speed of descent.
It is particularly advantageous if the opening orifices exposed by the control valve of the tilting drive system in the admission line and the discharge line are realized so that they correspond to the ratio of the surface area of the piston rod and the surface area of the piston of the hydraulic cylinder of the tilting drive system. It thereby becomes possible in a simple manner to specify different hydraulic flows for the piston side and the piston rod side of the double-action hydraulic cylinder.
In one advantageous embodiment of the invention, in the delivery line downstream of the control valve of the elevating drive system and upstream of the control valve of the tilting drive system, there is a check valve that opens toward the control valve of the tilting drive system. When the elevating drive system is actuated to raise a load, with a simultaneous actuation of the tilting drive system, operating conditions can sometimes occur in which hydraulic fluid flows from the tilting drive system to the elevating drive system, if the elevating drive system is supporting a lower load than the tilting drive system. The result can be a direction of movement that is opposite to the desired direction of movement of the tilting drive system. During such operating conditions, the check valve in the delivery line of the pump prevents hydraulic fluid from flowing back to the elevating drive system, and thus has the function of a load maintenance valve for the tilting drive system.
In one embodiment of the invention, there is at least one additional drive system, in particular for the drive system of a side loader, which drive system is realized in the form of a double-action hydraulic cylinder and can be actuated by means of a control valve, in particular by means of an electrically actuated control valve that is connected downstream of the control valve of the tilting drive system to the pump line, whereby a load pressure signal line is connected downstream of the admission-side throttle cross section of the control valves, and the load pressure signal lines are connected by means of a system of shuttle valves to a common load pressure signal line which is connected to a pressure balance. The load pressure signal line of the elevating drive system can thereby be connected to the control pressure line that leads to the control surface of the descent braking valve that is active in the closed position, which control pressure line is connected between the control valve and the check valve to the hydraulic line that leads to the lifting cylinder. On the tilting drive system, the load pressure signal line can be connected to the control pressure line that leads to the control surface, i.e. the one that is active in the open position, of the flow regulator that is located in the admission line.
The pressure balance appropriately connects the delivery line with the tank line and has a closed position and an open position, whereby the pressure balance is pressurized toward the open position by the pump pressure and toward the closed position by the highest load pressure of the actuated users applied to the common load pressure signal line, as well as by a spring. The pressure balance thus ensures that only the hydraulic flow required by the users flows to the users, and the additional amount of hydraulic fluid delivered, e.g. by a constant velocity pump, can flow back to the reservoir. When the users are not actuated, the pressure balance ensures the unpressurized circulation of the hydraulic fluid delivered by the pump.
In one preferred embodiment, the actuator device of the control valve of the tilting drive system and the actuator device of the control valve of the additional drive system are realized in the form of a double-action proportional magnet, whereby the control valves are centered in their middle position by means of a spring device. Such double-action proportional magnets take up significantly less space than two separate proportional magnets for the deflection in both directions of a control valve of a double-action user.
In one particularly advantageous embodiment of the invention, the control valve of the elevating drive system and/or the control valve of the tilting drive system and/or the control valve of the additional user and/or the check valves and/or the pressure relief valve and/or the flow regulators and/or the shuttle valves and/or the pressure balance are located in a control block that has a multi-layered construction consisting of a plurality of segmental plates that are connected to one another in a laminated fashion and have recesses, the contours and location of which in relation to one another form hydraulic fluid channels and housing borings, as well as control chambers. The recesses in the segmental plates can be manufactured by a laser cutting process, for example, or by stamping. Such a control block consisting of a plurality of segmental plates, in which the individual segmental plates are soldered to one another, for example, is significantly faster, easier and cheaper to manufacture than a conventional control block manufactured using casting technology. In addition, the channels, borings and control chambers for the control valves can be created in a simple manner by the recesses in the segmental plates in the control block. For this purpose, only the valve seats of the seat valves and the housing borings that hold the control slides of the longitudinal slide valves in the control block need to be machined. On longitudinal slide valves, the machining activities can be limited to a single fabrication step, e.g. to a honing process. In addition, the control block takes up significantly less space than conventionally manufactured control blocks, because there is no need to leave room for the minimum orifices required for casting the channels and borings. As a result, there is also a significant reduction in the weight of the control block.
The invention teaches that it is particularly advantageous to locate the connections for the pump line and the tank line as well as the connections for the hydraulic lines leading to the users on one surface of a segmental plate that forms a lateral surface of the control block, and to locate the electrical actuator devices of the control valves on the opposite lateral surface of the control block that is formed by a surface of an additional segmental plate. The user connections as well as the actuator devices are thus formed on opposites sides of the control block, each of which is formed by a surface of a segmental plate. Consequently, the control block takes up a particularly small amount of space, because it is no longer necessary to machine the edges of the segmental plates and likewise it is no longer necessary to machine the edges of the lateral surfaces of the laminated control block that is formed by the edges of the segmental plates.
In one embodiment of the control block, the connections for a pump connection and a tank connection as well as the connection sockets for the user connections are soldered into the control block. The connection sockets, which have threaded sections to receive lines or hoses, can be realized in the form of prefabricated parts that can easily be soldered into corresponding borings of the segmental plates.
It is also particularly appropriate if the actuator devices of the control valves are fastened in housing components that are soldered into the control block. The housing components, which have corresponding threaded connections for the fastening of the electrical actuator devices, can therefore also be manufactured separately, and can be easily soldered into borings in the segmental plates.
It is also particularly advantageous if the housing borings of the control valves and/or the housing boring of the pressure balance and/or the housing boring of the pressure relief valve and/or the housing borings of the check valves and/or the housing borings of the flow regulators and/or the housing boring of the descent braking valve and/or the housing boring of the shuttle valves, all of which are located in the control block, can be closed by means of housing components that are soldered into the control block. The housing borings, which also form control pressure chambers for the corresponding valves, can thus be closed in a simple manner.
The result is that the control block can be manufactured more easily and more economically, because no covers or plugs are necessary for the control pressure chambers or to close the housing borings, which covers or plugs would have to be fastened to the control block by means of corresponding threaded connections.